Fossils and Geologic Time

What fossils are NOT…

My doctor says I’m in the early stages of fossilization!

What do you know? Is it a fossil?

NO

YESThese are cretaceous microfossils!

Fossils?

Fossils…are:

• Preserved remains or traces of an organism that lived in the past.

•Fossils are formed when organisms die and are buried in sediment. Eventually the sediment builds up and hardens to become sedimentary rock.

Interpreting Environments

• Fossils can be used to interpret and describe ancient environments.

graptolites

gingko

trilobite

Fossils…what are they for?

• Specific conditions are needed for fossilization.

• Only a tiny percentage of living things became fossils.

Where do fossils form? What can become a fossil?

Kinds of fossils

1. Petrified - when minerals replace the remains and they become rock

2. Mold - when the shell remains and the contents dissolve (hollow)

3. Cast - when the mold becomes filled with minerals that are not a part of the original organism

PETRIFIED MOLD

CAST

So…fossil or not?

• Sometimes whole animals become preserved intact, but this is very rare. If an organism is surrounded by ice or tar they might be discovered looking much the same as they did when they died.

AETOSAUR

FOUND IN THE NATIONAL PETRIFIED FOREST

Intact Fossils

Some remains can be preserved in ice…

Fossil Record• The dating of all

fossils is included in the Geological Time Scale. This scale divides the time that the earth has existed into 4 eras.

* Eras are then divided into periods based on common events in that time period.

Geologic timeGeologic time

The relative geologic time scale has a sequence of

Relative Geologic Time Scale

eons eras periods epochs

But only approximate numbers indicating how long ago each of these times occurred.

Let’s start at the beginning…

What would the rocks say if they could talk?

Pre-Cambrian

• Began with the formation of the Earth 4.6 billion years ago.

Bacteria appeared 3.5 billion years ago, followed by algae and fungi.

Paleozoic Era• Divided into 5 periods:

Cambrian period - Sponges, snails, clams and worms evolve

Ordovician period - First fishes evolved and other species become extinct

Silurian period - Land plants, insects and spiders appear

• Devonian period - Amphibians evolve and cone-bearing plants start to appear.

Carboniferous period - Tropical forests appear and reptiles evolve.

Permian period - Seed plants become common and insects and retiles become widespread. Sea animals and some amphibians begin to disappear.

Next…The Big Guys…

Mesozoic Era• Divided into 3 periods:

Triassic period - Turtles and crocodiles evolve and dinosaurs appear.Jurassic period - Large

dinosaurs roam the world. First mammals and birds appear.

Cretaceous period – warm, reptiles, small dinosaurs and flowering plants appear.

As evolution occurred over time…

New species began to appear…

Cenozoic Era• Divided into 2 periods:

Tertiary period - First primates appear and flowering plants become the most common.

Quaternary period - Humans evolve and large mammals like woolly mammoths become extinct.

So, In summary:

So, how do we sort out all of the fossils?

How do we know which fossils are from which part of the Geologic Time Scale?

1. Relative Dating*Law of Superposition*Index fossils*Correlation of layers

2. Absolute Dating (Radioactive dating)

Relative Dating Relative dating: looks at where the fossil is

located to determine its age relative to other fossils. This only works if the area has been undisturbed.

Law of Superposition:

– Nicolas Steno (1638-1686)– In an undisturbed succession of sedimentary rock

layers, – the oldest layer is at the bottom – and the youngest layer is at the top

• This method is used for determining the relative age of rock layers (strata) and the fossils they contain

Index fossils:• Index fossils can provide the relative age of a rock layer. • Existed only during specific spans of time• Occurred in large geographic areas

Index fossils include fusulinids and trilobites.

How do we know if we have the right fossils put together?...

Answer: Correlation of layers…

Faunal succession (correlation by fossils)

Relative age: Correlation How is this done?

index fossil short-lived organism; points to narrow range of geologic time

Fossil species succeed one another through the layers in a predictable order

fossil assemblage group of fossilsassociatedtogether

Correlation helps to determine the age of the fossils and their geologic time

Correlation: We can match up layers with similar fossils at different places by index fossils and comparison.

Absolute Dating• Uses radioactive elements near the fossils

to determine the actual age of the fossils.•By determining the age of the radioactive element, scientists can calculate the age of the fossil buried nearby.

The absolute age of fossils is estimated by dating associated igneous rock and lava flows.